Year? and what is it worth?

My WWII veteran father-in-law (just passed away) willed me his somewhat old Stevens Model 311 shotgun. I can not find a serial number on it anywhere. It is a 15ga side by side made by Savage Arms. Any idea what it is worth and the year it was made?
New to this forum, so take it easy on me!!

P.S. the gun is in fairly good shape..I would give it a 7 on a 1-10 scale.

No way am I getting rid of it, just need to insure it. My 13yr old son will acquire it hopefully years down the road.

I will post picture this evening. After further review....it is a 16ga. I used a large magnifying glass and the 6 was stamped poorly. I was going by what my wife was telling me. Should of known better.